In addition to being important for normal fetal and postnatal growth, there is increasing evidence that insulin-like growth factors I and II (IGF-I, IGF-II) also support the growth of certain cancers. Biologic responses to IGF-I and IGF-II are signaled by the IGF-I receptor. Therefore, we are focusing our research effort on understanding signaling by the IGF-I receptor and the development of reagents that block IGF-I receptor function.We have used the yeast two-hybrid system to identify new binding partners for the IGF-I receptor. Last year, we reported that SOCS-2 (suppressor of cytokine signaling) was identified as a binding partner for the IGF-I receptor in a two-hybrid screen of a human fetal brain library. To determine whether or not other members of the SOCS family interact with the IGF-I receptor, SOCS-3 cDNA was cloned by utilizing the polymerase chain reaction. In the yeast two- hybrid system, SOCS-3 interacts with the autophosphorylated IGF-I receptor but not with the kinase negative receptor construct. In contrast, in in vitro experiments utilizing GST-SOCS-3 and IGF-I receptor from fibroblasts, and in transient transfection experiments in 293 human embryonic kidney fibroblasts, SOCS-3 binds to both the basal and activated receptor. SOCS-3 is weakly phosphorylated on tyrosine residues in an IGF-I dependent fashion and is more heavily phosphorylated on serine residues with or without stimulation of 293 cells with IGF-I. SOCS-3 is ubiquitinated in 293 cells and immunostaining shows that FLAG-SOCS-3 is localized in both the cytoplasm and nucleus. In addition , we have evidence that SOCS-3 can exist as a dimer in transfected 293 cells and self-associates in a yeast two-hybrid assay. These new findings expand the possibilities for the function of SOCS-3 in the cell. We have transfected IGF-I receptor negative fibroblasts with two truncated IGF-I receptor constructs, one consisting of the transmembrane and cytoplasmic domain of the IGF-I receptor and the second consisting of the transmembrane domain of oncogenic erbB-2 (neu) and the cytoplasmic domain of the IGF-I receptor. Clones from vector only transfection showed no anchorage independent growth whereas the truncated wild type IGF-I receptor clones and the truncated neu/IGF-I receptor clones showed many large colonies in the soft agar assay. In contrast, the proliferation of the two receptor clones in medium containing low serum was no different than the vector only clones. These results demonstrate a discordance between proliferation and anchorage-independent growth and provide support for the existence of a distinct transformation pathway emanating from the IGF-I receptor. - IGF-I receptor, SOCS-3, erbB- 2(neu),